A condenser microphone is provided with a condenser microphone unit (an electrostatic acoustic-electric converter) including a diaphragm and a backplate disposed opposedly, and the output impedance of the condenser microphone unit is very high. Therefore, the condenser microphone unit incorporates an impedance converter for converting impedance to low one. In many cases, as the impedance converter, a field effect transistor (FET) has been used.
Usually, the condenser microphone is used by being connected to a phantom power source via a balanced shielded cable. Therefore, the condenser microphone is provided with an output connector in the rear end part of a microphone housing thereof.
As the output connector, an output connector specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Audio Equipment” is used. This output connector has three terminal pins, No. 1 pin being used as grounding, No. 2 pin as the hot side of signal, and No. 3 pin as the cold side of signal.
As described above, most of the condenser microphones are used by being connected to the phantom power source via the balanced shielded cable. However, the microphone used outdoors frequently, such as a superdirectional gun type condenser microphone, incorporates a battery serving as a driving power source.
In the microphone incorporating a driving battery, a battery holder for accommodating an AA size battery or the like is provided between the microphone housing having the microphone unit and the output connector, and a cylindrical battery cover is slidably put on the battery holder (for example, refer to Japanese Patent Application Publication No. 2006-33463).
As measures against noise caused by electromagnetic waves, the microphone housing including the output connector part is required to have high shieldability. However, the battery built-in type condenser microphone has a movable part, the battery cover, as described above, and the grounding thereof is difficult to do, so that a high-frequency current caused by external electromagnetic waves (especially, strong electromagnetic waves emitted from a cellular phone) intrudes into the microphone housing through a contacting part of the movable part and a fixed part, whereby the high-frequency current is detected by the FET, and noise is generated easily.
Therefore, the invention described in Japanese Patent Application Publication No. 2006-33463 discloses a method in which with the insertion of a plug of a microphone cord (balanced shielded cable) into the output connector, a plate spring comes into contact with the battery cover, thereby making electrical continuity.
However, in this method, since the contacting part is a point contact, a sufficient effect cannot sometimes be achieved in inhibiting the high-frequency and high-output electromagnetic waves emitted from the cellular phone.
The invention described in Japanese Patent Application Publication No. 2006-2033407 proposes a technique in which a conductive cloth is disposed between the battery cover and a connector sleeve of the output connector to make electrical continuity.
According to this technique, since the battery cover and the connector sleeve of the output connector are in planar contact with each other via the conductive cloth, noise caused by electromagnetic waves can be prevented. However, problems to be solved remain still.
One of the problems is that when the battery is exchanged, the battery cover is slid to the output connector side to open the battery holder, and at this time, if a coming-off preventing device for the battery cover is not provided, the battery cover comes off inadvertently. If the coming-off battery cover collides with a floor surface or the like and is deformed, the battery cover becomes unusable.
Conventionally, therefore, a stopper ring that comes into contact with the end face of the connector sleeve has been provided on the inner peripheral surface of the battery cover to prevent the battery cover from coming off. However, the provision of the stopper ring separately from the conductive cloth increases the number of parts, which is unfavorable in terms of cost. Also, it increases the manpower for assembling work, which is also unfavorable in terms of productivity.
Accordingly, an object of the present invention is to provide a condenser microphone having a battery holder and a battery cover (movable part) therefor, in which one part is used as a member for grounding the battery cover and as a member for preventing the battery cover from coming off.